1. Field of the Invention
The present invention relates to switching power supply apparatuses, and more particularly, to a switching power supply apparatus used for turning on a high-voltage discharge lamp operated by applying direct current.
2. Description of the Related Art
Switching power supply apparatuses including a DC-DC converter as a fundamental circuit are available and various power supply apparatuses for a high-pressure discharge lamp have been designed, for example. Note that, in the following description of a known power supply apparatus, only components which relate to the present invention are briefly described and the complete detailed description of the entire apparatus is omitted hereinafter.
FIG. 3 is a circuit diagram of a power supply apparatus 30 including a DC-DC converter disclosed in Japanese Unexamined Patent Application Publication No. 2003-189602 (Patent Document 1).
In the power supply apparatus 30 disclosed in Patent Document 1, a MOSFET Q1 defining a switching element, a resonant coil L2, and a choke coil L1 are connected to one another in series between one end of a direct current power supply Vin and one end of an output terminal Po. A rectifier diode D1 is connected between one end, which is closer to the resonant coil L2, of the choke coil L1, the direct current power supply Vin, and the other end of the output terminal Po. The power supply apparatus 30 is configured such that a series circuit including the resonant coil L2 and the rectifier diode D1 connected to each other in series is connected in parallel to a series circuit including a capacitor C5 and a MOSFET Q2 defining a switching element connected to each other in series. The power supply apparatus 30 includes a control circuit 11 for controlling the MOSFET Q1 and the MOSFET Q2 so as not to be turned on simultaneously, but rather, to be turned on alternately with a predetermined dead time.
The power supply apparatus 30 may be controlled to be selectively switched between an operation mode and a standby mode in accordance with a power supply operation control signal supplied from a system in which the power supply apparatus 30 is included. In the standby mode, control is typically stopped, after the MOSFET Q1 is controlled to be turned off and the MOSFET Q2 is controlled to be turned on.
However, in the standby mode of the known power supply apparatus 30, when the MOSFET Q1 is damaged due to a short circuit, the following problems arise.
In the standby mode, since the MOSFET Q2 is normally turned on, a connection point of the capacitor C5 and the MOSFET Q2 has a potential substantially the same as the ground potential. Therefore, in the power supply apparatus 30 shown in FIG. 3, when the MOSFET Q1 is short-circuited, that is, when the MOSFET Q1 is turned on as described above, an input voltage Vin is directly applied to the capacitor C5. Since the capacitor C5 is not designed to have a large amount of voltage is applied thereto, a capacitor having low voltage resistance is normally used as the capacitor C5. Accordingly, the capacitor C5 may be damaged when an input voltage Vin is directly applied thereto.
If voltage resistance and allowable current of an element that may be damaged as described above are increased, the probability of the damage is reduced. However, the cost and size of each element is increased.